1. Field of the Invention:
The present invention relates to an arrangement for transmission of motion. It concerns more particularly the field of speed reducers in which the high speed motion of an input shaft is transformed to low speed of an output shaft by means of systems of shafts and gears.
2. Description of the Prior Art:
Speed reducers may constitute independent mechanical devices and may also be included in more complex mechanisms. In general they are formed by the association of a shaft carrying a pinion with a small number of teeth engaging a pinion with a larger number of teeth carried on a second shaft, which also carries a pinion with a small number of teeth which engages in turn a pinion with a larger number of teeth carried by a third shaft and so forth, the number of shafts and pinions being a function of the speed-reduction ratio desired.
The main drawback of such reducers is their bulkiness. This is due to the fact that the interior volume of the reducer is poorly utilized because of the spacing required between the driving and driven gears. Another reason for this bulkiness is the fact that there are unavoidable flexure forces on the various shafts; these must therefore be very large in diameter and supported by large bearings.
In order to save space, planetary gear reducers are often used. They are characterized by the fact that they can have several "planets" driven by a single "sun". The planet wheels being evenly spaced around the sun wheel, the flexure forces on the input shaft are zero and, consequently, it can have a small-diameter shaft and low-capacity bearings. The same is true of the output shaft. Moreover, the planet wheels being disposed concentrically with the drive gear, which itself is concentric with the annulus, in most designs, reducers with planetary trains are very compact.
Some well-known designs permit the realization of very large reduction ratios with a small number of gears. These trains have four disadvantages: first, they require great precision in construction so that all planet wheels participate in transmission. If the precision is insufficient, just one will carry the load, and this diminishes considerably the load capacity of the reducer and, consequently, removes all its attractiveness. Next, particularly in reducers with large reduction, internal clearances and especially those between gear teeth may result in a large angular play between the input and output shafts. Decrease of these clearances leads to a considerable increase in the cost of the reducer but may still not be very effective in most cases.
It can also be shown that in certain types of epicyclic reducers there is a "recirculating" power which would not be troublesome in itself if the energy losses of the reducer were not proportional to the actual delivered power. Now, such is not the case and for certain configurations this "recirculating" power is much greater than the actual power. Consequently the losses are elevated and the efficiency of the reducer is low. Finally, certain configurations, unfortunately those with high reduction ratios, lead necessarily to a high drive-shaft inertia. This inertia is referred to the output shaft.
As is well-known, the inertia reflected to the output shaft is equal to the drive shaft inertia multiplied by the square of the reduction ratio. A very important consequence of this is that, if the reducer drives a mechanism that can be accidentally blocked, the kinetic energy of the input shaft is such that it can not be absorbed by an elastic deformation of the elements "downstream". There then occurs a plastic deformation or rupture and therefore destruction of the reducer and/or the driven mechanism. Also known are reducers called "deformation wave" reducers (in English: "strain wave" gearing) in which one makes use of the elastic deformation of a "bell" with a given number of teeth on its periphery which is constrained by an elliptical ball bearing situated at its center to mesh with a fixed annulus having on its inner cylindrical surface a larger number of teeth than the "bell". The result is a relative motion with a high speed ratio between the input shaft, which drives the inner elliptical race of the ball bearing, and the output shaft which is driven by the bell. Moreover, this high ratio is obtained with just two toothed parts.
At first sight, then, this reducer seems very attractive. Several drawbacks prevent its use in many cases. First, it has low torsional rigidity so that the bandpass of a machine using it is very often isufficient. This lack of rigidity cannot be compensated since it is a consequence of the very principle used in the device. It is due to, among other things, the elastic compression of the balls of the elliptical bearing between the races and also to their play. Another important drawback is the high cost of the reducer, mainly for the elliptical ball bearing.
Finally, its principal disadvantage is the enormous primary inertia of the input shaft which drives the inner race, necessarily of large diameter, of the elliptical bearing. The result of this primary inertia is to limit accelerations of the input shaft and also to make unavoidable the destruction of the reducer and/or the driven device in case of sudden stoppage of the latter.
There are different versions of these "deformation wave" reducers. In some the elliptical bearing, and thus its drawbacks, has been eliminated, but none reduce the primary inertia significantly. It must be said that all these reducers are reasonably compact, although less so than the standard epicyclic reducers, and their operating clearances are rather small.
Finally, numerous other types of reducers are known but all have one or more of the disadvantages noted above. The present invention permits avoiding these drawbacks.
The improved speed reducer of the present invention is remarkable in that its input shaft carries a gear which drives a minimum of two and at most three planet wheels which, in turn, drive a single deformable annulus with an axis concentric with the axis of the input shaft, the teeth of the gear having a primitive diameter less than the diameter of the circle circumscribed around the set of the planet wheels in operating position and meshing without play with the teeth of the input shaft, the difference between these two diameters being at least sufficient for the meshing to be without play for the entire life of the speed reducer.
According to a first embodiment of the invention there is mounted around the deformable annulus a rigid ring the inner diameter of which is greater by some hundredths of a millimeter than the diameter of the circle circumscribed about the deformable annulus after it has been mounted around the planet wheels surrounding the input shaft.
According to a second embodiment of the invention, the rigid ring is mounted floatingly around the deformable annulus and its axial displacement is limited on one side by a shoulder on the housing and on the other by a brace.
According to another embodiment of the invention, the input shaft, the planet wheels and the deformable annulus form a simple epicyclic train in which the deformable annulus is immobilized in rotation, while the axles of the planet wheels are mounted on a single plate fastened to the output shaft of the reducer.
According to a variant of the invention, the plate holding the planet wheel axles receives in turn at its center a drive gear for another reducer stage such as described above.
Advantageously, each planet turns by the intermediary of a bearing on a race which, by means of a hub, is centered with play about the axis of the planet, the hub receiving along a diameter of its boring a thin plate attached by its two ends to the boring and rigidly fastened at its center in a diametral slot of the planet wheel axle, the orientation of the slot parallel to the reducer axis being preferably essentially perpendicular to the radius from the center of the planet-holding plate and to the center of each planet axle.
Advantageously again, the thin plate has axial grooves which increase its flexibility without significantly decreasing its rigidity to traction and compression.
According to another embodiment of the invention, the planet-supporting hub and the planet axle are held together in their displacements with the help of radial grooves carried by one of the two elements while the other has radially oriented projections so that the planet wheel can be displaced only radially.
According to another embodiment of the invention, the bearing between a planet wheel and the race holding it is of the needle type which may be prestressed.